Mass spectrometry (MS) is a useful analytic technique for identification of chemical structures, determination of components of mixtures, and quantitative elemental analysis. This analytical technique is based on the separation of the ionized components of an analyte by their mass-to-charge ratios. Often, in either the collection or ionization stage of a sample for analysis, an undesired species can be present at a very high level in the sample. Examples of undesired species include the background helium carrier gas when using a gas chromatograph column as the input to the mass spectrometer and the residual argon gas found in samples obtained from inductively coupled plasma (ICP) sources. Thus, a mass filter that can selectively eliminate ions of a predetermined mass-to-charge ratio from an ion beam but fully transmit all other ions is desirable.
To this end, filters have been inserted into the path of an ion beam to remove target ions (such as a contaminant, or undesirable ion) of a specified mass-to-charge ratio while transmitting other ions. Preferably, the filter transmission function has a notch only one atomic mass unit wide to allow rejection of a single ion species. Such filters, made by using quadrupoles, have been reported in the literature.
A quadrupole filter is a device in which ions travel along an axis parallel to and centered between four parallel quadrupole rods connected to voltage sources (e.g., described in U.S. Pat. No. 3,334,225 (Langmuir) and No. 5,187,365 (Kelley)). FIG. 1 shows a typical quadrupole 10, which has four parallel, straight, (i.e., linear), elongated electrodes (or rods) 12, 14, 16, 18 connected to an oscillating voltage supply 20 that supplies a radio frequency (rf) oscillating voltage (hereinafter referred to as the "rf quadrupole voltage") to the electrodes. A pair of oppositely facing electrodes 12, 16 are connected to one pole and the other pair of oppositely facing electrodes 14, 18 are connected to the other pole of the voltage supply 20. The rf quadrupole voltage guides ions between the electrodes via well-known effective forces. (The rf frequency, represented by .OMEGA., of this rf quadrupole voltage is referred to as the "rf quadrupole frequency" hereinafter.)
As known in the art, to filter out an unwanted contaminant ion, a dipole field "excision" frequency is selected to correspond to the specific frequency of transverse motion that the undesired ion exhibits as it is guided down the quadrupole by the effective potential generated by the rf quadrupole voltage. This dipolar excision voltage (having a lower frequency than the rf quadrupole frequency) would coherently act to increase the transverse motion amplitude of the undesired ion as the ion traverses down the quadrupole. Eventually, the transverse motion amplitude becomes so large that the ion strikes the quadrupole structure and is eliminated from the ion beam. Other ions with different mass-to-charge ratios, due to their lack of synchronism with the excision frequency, would not increase their amplitudes in transverse motion significantly. In this manner, mass selectivity is achieved.
Thus, a notch filter is realized by operating a quadrupole in a rf-quadrupole-frequency-only configuration (i.e., no DC voltage, in which case the quadrupole acts effectively as an "ion pipe") and applying an oscillating dipolar excision voltage at a lower frequency than the rf quadrupole frequency to an opposing pair of the four quadrupole rods. Examples are found in Reinsfelder et al., "Theory and Characterization of a Separator Analyzer Mass Spectrometer," Int. J. Mass Spec. and Ion Physics, 37:241-250 (1981) and Miller et al., "A Notch Rejection Quadrupole Mass Filter," Int. J. Mass Spec. and Ion Physics, 96:17-26 (1990).
In such dipolar excision systems, the lower frequency dipolar excision voltage (creating a "dipole field") is applied to an opposing pair of the four quadrupole rods via an electronic coupling network. The reason such a coupling network is needed is that the higher frequency rf quadrupole voltage is applied such that any two adjacent electrodes are opposite in polarity, but the lower frequency excision voltage is applied such that the two oppositely facing electrodes to which this excision voltage is applied are opposite in polarity. Thus, the electronic coupling network is needed to isolate the excision voltage from the higher frequency rf quadrupole voltage. An example of such an electronic coupling network is described in "A Notch Rejection Quadrupole Mass Filter," Miller et al., supra (see FIG. 5 of Miller et al.). Such coupling networks require an additional radio frequency transformer to provide a means of isolating a single pair of rods out of the two pairs of quadrupole rods. The low frequency excision voltage is coupled via a primary winding on this transformer. This isolation scheme also requires the use of various radio frequency chokes and capacitors.